1. Field of the Invention
The present invention relates to a method of manufacturing a resin-layer coated carrier, a resin-layer coated carrier, a developer, a developing device and an image forming apparatus.
2. Description of the Related Art
With recent remarkable development of office automation equipment, image forming apparatuses such as a multi-functional peripheral, a printer and a facsimile apparatus that perform image forming processing by electrophotography have widely been spread. In an image forming apparatus employing electrophotography, a charging step, an exposure step, a developing step, a transfer step, a fixing step and a cleaning step are generally performed in order to form an image.
Specifically, firstly, a surface of a photoreceptor which is an image bearing member is uniformly charged in a dark place (the charging step), electric charges are removed by projecting signal light of a document image to the charged photoreceptor and an electrostatic charged image (electrostatic latent image) is formed on the surface of the photoreceptor (the exposure step). Then, a toner for development (hereinafter, simply referred to as “toner” unless otherwise specified) is supplied to the electrostatic charged image of the surface of the photoreceptor, a toner image as a visible image is formed (the developing step), this toner image is contacted to a recording medium such as paper and a sheet and to be performed with a corona discharge from the side opposite to the contact surface, an electric charge whose polarity is opposite to that of the toner is imparted to the recording medium, and thereby the toner image is transferred to the recording medium (the transfer step). Subsequently, the toner image on the recording medium is fixed by applying heat and pressure (the fixing step), and finally, toners left on the surface of the photoreceptor without being transferred to the recording medium is collected (the cleaning step).
An image forming apparatus employing an electrophotography forms a desired image on a recording medium by way of the above steps.
In such an image forming apparatus, a one-component developer containing only a toner or a two-component developer containing a toner and a carrier is used as a developer for developing a toner image. In the case of the two-component developer, functions including stirring, conveying and charging of toner particles are given by the carrier, and the carrier bears such functions, whereby controllability is improved compared with the one-component developer containing only a toner, and it is easy to obtain a high-quality image. Consequently, research and development of a carrier suitable for using together with a toner have been actively performed.
A carrier is composed of a core material and a coating resin, and has two basic functions of a function of stably charging a toner to a desired charge amount and a function of conveying a toner to a photoreceptor. Additionally, the carrier is stirred with a toner in a developer tank, then is conveyed onto a magnet roller, forms a magnetic brush, passes through a control blade, and returns into the developer tank again after developing the toner to be repeatedly used. In such repeated use, the carrier is required to stably exert the basic functions, especially, to stably charge a toner, in such repeated use.
Conventionally, with an aim of improving carrier characteristics of a carrier for electrophotography, surface modification processing has been performed to coat with a coating material the surface of a magnetic base particle which is a core material of the carrier. For controlling charging, mainly, coating with a resin is performed.
Typically, as methods for coating a carrier with a resin, a wet method for coating with a resin that is dissolved in a solvent and a dry method for coating with a resin particle with heat and impact force in a gas phase are used.
In many cases, resin coating of a carrier is performed by the wet method, in which specifically, an immersion coating method and a fluidized-bed spray coating method are included.
The immersion coating method is a method of immersing the magnetic base particle in a coating solution in which a coating resin is dissolved to be subjected to a coating treatment, followed by drying, however, since the magnetic base particle is immersed directly in the resin solution, it is easy to generate aggregation of the particles, and a yield of the coated carrier is significantly lowered.
The fluidized-bed spray coating method is a method of spraying a coating solution in which the coating resin is dissolved onto the surface of the magnetic base particle that is suspended in a fluidized bed (gas phase), followed by drying, however, since a solvent is used, it is easy to generate aggregation of the carriers, and the yield of the carrier is low. Further, there is also a problem such that the time required for manufacturing is long and productivity is low since a drying step is needed. Additionally, in this method, the thicker the resin coating layer is, the more the carriers aggregate, the yield is lowered, and the film thickness thus must be thinner. Therefore, such a carrier, in the case where the resin coating layer deteriorates due to long-term use, the magnetic base particle is exposed, and it is thus concerned that ability imparting charging to a toner is lowered.
For these wet methods, there is a problem that it is required to use a large amount of solvents dissolving the coating resin and environmental loads are high.
On the other hand, as a resin coating method of a carrier, a dry method without using a solvent is known (refer to Japanese Unexamined Patent Publication JP-A 2-87167 (1990)). The dry method is a coating method of causing a fine resin particle to adhere onto the surface of the magnetic base particle by mixing and stirring without using a solvent and spreading the adhered fine resin particle by plastically deforming with mechanical impact force.
According to this method, it is hard to generate aggregation of the carriers, and it is possible to obtain the resin coated carrier at a high yield even when the film thickness is thickened. Further, there is an advantage that the time required for coating is significantly shortened since the processing of cleaning, drying and the like is not necessary, and the productivity is high. Moreover, since there is no need for facilities for collecting or burning solvents, it is possible to reduce production costs.
However, the fine resin particle used for the dry method, as disclosed in JP-A 2-87167, is an acrylic resin fine particle or a styrene-acrylic resin fine particle with high charging ability, and there is a problem that these fine particles tend to have higher surface energy, therefore surface contamination of a coated carrier is easily developed by a toner particle, and the ability imparting charging of a carrier for a toner is lowered due to the toner spent with long-term use. Additionally, many of such fine resin particles to which carriers do not adhere remain so that fine powder thereof causes a charge amount to lower.
As a resin coating method of a carrier, it is possible to use a dry method of causing a fine resin particle to adhere onto the surface of the magnetic base particle by mixing and stirring without using a solvent and spreading the adhered fine resin particle by plastically deforming with mechanical impact force.
According to this method, it is hard to generate aggregation of the carriers, and it is possible to obtain the resin coated carrier at a high yield even when the film thickness is thickened. Further, there is an advantage that a step required for coating is cut since the processing of cleaning, drying and the like is not necessary. Moreover, since there is no need for facilities for collecting or burning solvents, it is possible to reduce production costs.
However, in this method, many fine particles which remain without being immobilized are on the surface of the carrier, which causes deterioration of the carrier characteristics. Additionally, there is a problem that it takes a long time to perform a process for eliminating the remaining fine particles. Therefore, in the method disclosed in JP-A 2-87167, it is needed to adjust the carrier characteristics by changing characteristics of the resin used for coating so that the selection of resins is significantly restricted.